(a) Field of the Invention
The present invention relates to a method and apparatus for transmitting and receiving a reference signal using an unlicensed band in a mobile communication system.
(b) Description of the Related Art
3rd generation partnership project long term evolution (3GPP LTE) has been globally enlarged to 251 commercial networks in 93 countries for about 4 years after being commercially available on 2009 and recently, the number of subscribers throughout the world has exceed one hundred million and 2 ten million. Accordingly, use of data through an LTE service has been further rapidly increased as a pad type terminal for supporting a large screen and high resolution has been widely used, and in the future, if device communication such as machine-to-machine (M2M) is extended, it is expected that “data explosion” will occur in a cellular network.
In order to accommodate explosive increase of data as described above-wireless communication providers have conducted researches into a method for remarkably extending a data capacity to be provided by a network without investment of large scale equipment or broadscale increase in communication costs while sufficiently meeting demands for data use of consumers. As one of solutions that have been introduced by a plurality of wireless communication providers in order to correspond to explosive increase of data in a cellular network, wireless data traffic concentrated on a 3G or LTE network is dispersed (that is, off-loaded) using a wireless local area network (WLAN) communication method that is managed in an unlicensed band such as an industrial, scientific and medical (ISM) band. An unlicensed band is not a band in which a communication provider is capable of ensuring exclusive frequency license and thus, when an unlicensed band is used, a significant level of traffic capacity may be dispersed at a low cost. However, since a large amount of communication apparatuses are capable of being used without limitations only when interference relative regulation within a band is observed in an unlicensed band, it is difficult to ensure communication quality corresponding to a level provided through a communication service through a licensed band in which exclusive license is ensured.
3GPP has progressed standardization for association with Wi-Fi as the IEEE standard technology as well as high speed packet access (HSPA) or an LTE system from 2002, and currently, discussion and standard for various types of interworking methods have been established. However, it is not easy to integrate two systems into one network in order to ensure effective mobility and quality of service (QoS) due to a structural difference between a cellular network and a wireless local area network (WLAN). In order to overcome this difficulty, recently, LTE on unlicensed spectrum (LTE-U) or U-LTE technologies have been actively discussed. Recently, 3GPP has held workshop for standardization for cellular technologies in an unlicensed band and discussed requirements based on opinion of each vendor. In addition, 3GPP has approved the “Study on Licensed-Assisted Access to Unlicensed Spectrum” as a study item (SI) for providing an LTE service in an unlicensed band and has standardized the SI.
Recently, LTE has supported a frequency bandwidth of 1.4 MHz to 20 MHz (1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, and 20 MHz), and mobile communication providers select a bandwidth appropriate for a provider in an allocated frequency bandwidth and provide an LTE service.
3GPP LTE LAA that has currently progressed standardization basically considers a system bandwidth of 20 MHz and does not consider use of a system bandwidth less than a minimum of 5 MHz (i.e., 1.4 MHz and 3 MHz). This refers to a bandwidth between 5 GHz and 20 MHz in a frequency bandwidth considered in the legacy LTE. That is, an available unlicensed band frequency may be classified into 5 MHz, 10 MHz, 15 MHz, 20 MHz, and so on. In addition, it is possible to use a frequency of several tens of MHz or several hundreds of MHz for each area in an unlicensed frequency bandwidth of 5 GHz. Accordingly, an unlicensed band frequency of a band of 5 GHz may include a plurality of frequencies with a fixed size of bandwidth. For example, in the case of Korea, there is an unlicensed bandwidth of 150 MHz in 5.1 GHz to 5.25 GHz, and when an unlicensed bandwidth of 150 MHz is divided into a size of 20 MHz, and about 7 independent carriers may be used.
The legacy LTE sets a frequency bandwidth of a fixed size for each cell and provides a service according to frequency allocation policy. This is because mobile communication providers are capable of exclusively using a corresponding frequency bandwidth, and mobile communication providers may optimize a mobile communication network by changing a predetermined frequency bandwidth as necessary. However, in an unlicensed band of 5 GHz, unlicensed devices using different wireless access technology such as WiFi as well as an LAA device coexist, and thus interference and load degrees may be changed according to degrees of access and use of these devices. Accordingly, the LAA system needs to dynamically use an unlicensed band frequency in consideration of interference and load degrees in order to increase unlicensed band frequency utilization efficiency and to effectively transmit/receive data, and to this end, there is a need for a method for selecting and changing a carrier in order to dynamically manage carrier in the LAA.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.